1. Field of the Invention
The present invention relates to a fuel cell system, including a fuel cell, which is operated to provide power generation by consuming a fuel gas supplied to an anode and an oxygen-containing gas supplied to a cathode. Further, the present invention relates to a method of operating such a fuel cell system.
2. Description of the Related Art
A polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly), which includes an anode (fuel electrode), a cathode (air electrode), and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is made up of a polymer ion exchange membrane. The membrane electrode assembly, along with separators sandwiching the membrane electrode assembly therebetween, collectively make up a unit of a power generation cell (unit cell) that generates electricity. Generally, when placed in use, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to a fuel gas flow field. The fuel gas flows through the fuel gas flow field and along the anode. A catalyst included in the anode induces a chemical reaction in the fuel gas, in order to split hydrogen molecules into hydrogen ions and electrons. The hydrogen ions move toward the cathode through a suitably humidified electrolyte membrane, whereby the electrons flow through an external circuit to the cathode, thus generating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas, such as air, is supplied to the oxygen-containing gas flow field, wherein the oxygen-containing gas flows along the cathode to induce a reaction. In the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water.
The efficiency of this type of the fuel cell tends to change easily depending on ambient pressure. For example, if the fuel cell is operated in a high pressure environment, power generation by the fuel cell is carried out efficiently. A compressor for supplying air to the cathode operates highly efficiently when the fuel cell is operated within a predetermined compression ratio range. However, at high altitudes, where atmospheric pressure is low, since the atmospheric air density is low, the compression ratio must be increased, thus lowering the efficiency of the compressor.
In this regard, Japanese Laid-Open Patent Publication No. 2001-345112 discloses a fuel cell system, as shown in FIG. 6. The fuel cell system includes a fuel cell 1, a fuel processor 2 for supplying hydrogen to an anode (not shown) of the fuel cell 1, a compressor 3 for supplying oxygen to a cathode (not shown) of the fuel cell 1, a pressure regulator 4 for changing the back pressure of a cathode tail gas, pressure detecting sensors 5a to 5d, and a controller 6.
The pressure sensor 5a detects an inlet pressure of the compressor 3, whereas the pressure sensor 5b detects an outlet pressure of the compressor 3, i.e., the pressure occurring along a line 7. The pressure sensor 5c detects a pressure along another line 8, which is the back pressure applied to the system by the pressure regulator 4. The pressure sensor 5d detects an ambient pressure (atmospheric pressure) in the vicinity of the fuel cell system.
In the above configuration, it is desirable to maintain the outlet pressure of the compressor 3 at a substantially constant level, irrespective of the ambient pressure. Therefore, when the fuel cell system is operated at a high altitude, such as at the peak of a mountain, the pressure sensor 5b monitors the pressure along the line 7. As the pressure along the line 7 begins to decrease commensurate with the increase in altitude, the pressure sensor 5b outputs a signal to the controller 6, whereby the controller 6 outputs a signal to the pressure regulator 4 for increasing the system back pressure up to a desired pressure value.
However, in the aforementioned conventional technique, a pressure regulator 4 must be provided, and further, the valve position of the pressure regulator 4 must be adjusted in correspondence with fluctuations in atmospheric pressure. Thus, controlling the pressure regulator 4 through the controller 6 is quite complicated.